FIG. 1. illustrates one embodiment of a minimally invasive surgical device 20. This particular device 20 is a suturing device having an end effector 22 that includes a needle 24. The needle 24 is coupled to an actuator 26, in this example, a handle which pivots on a pivot point 28 in the housing 30. When the actuator 26 is squeezed in the direction of arrow 31 a handle 32 which is part of the housing 30, a needle tip 34 of the needle 24 will move in a distal direction 36. A tolerance stack 38 is schematically illustrated to account for the fact that there are many different tolerances which can cause the amount of distance which the needle tip 34 moves to vary. The suturing device 20 is equipped with a ferrule 40 which lies at the end of a suture 42. Ideally, the actuator 26 moves the needle tip 34 across the tissue bite area 44 and into contact with the ferrule 40, such that the ferrule 40 is engaged by interference contact with the needle tip 34 without undesired deformation of the ferrule 40. The actuator 26 can then be returned to its starting position by spring 46, causing the needle 24 to draw the ferrule 40 and its suture 42 back through the tissue bite area 44.
FIGS. 2A and 2B are enlarged views of the distal end of the device 20 from FIG. 1 illustrating what can happen if the needle tip 34 is advanced too far. In this case, as demonstrated in FIG. 2A, the interference of the needle tip 34 and the ferrule 40 causes the ferrule to flare out as the larger portion of the needle 24 is driven into contact with the ferrule 40. When the needle 24 tries to move back in a proximal direction 48, as shown in FIG. 2B, the flared ferrule 40 contacts a release spring 50 when it is not supposed to. Depending on how tightly the needle 24 is wedged into the ferrule 40, this can jam the device or cause the ferrule 40 to release from the needle 24 when such release is not desired. In either case, the device does not work properly. The tolerance stack can vary from unit to unit, so it would be advantageous to have a device and method to compensate for the effects of the tolerance stack, thereby ensuring reliable device operation.